The present invention generally relates to matrix materials for use in a wide variety of end use fields and applications. More particularly, the invention relates to new and improved self-repairing, settable or curable matrix material systems containing so-called smart-release fiber reinforcements, alone or in combination with other reinforcement. My prior parent application, Ser. No. 540,191, filed Jun. 19, 1990, describes the new and improved inorganic and organic matrix composites employing concrete matrix systems and asphalt matrix systems as illustrative embodiments. That prior application describes smart-release hollow fiber a additives in settable construction materials and thermoplastic matrices, such as asphalt. This application is being filed to describe other embodiments of the smart-release matrix composite materials generally described in my earlier application and to provide additional examples of end use applications to which these new and improved compositions, articles and methods may be specially adapted and used.
Cement is a fine, gray powder consisting of alumina, lime, silica and iron oxide which sets to a hard material after mixture with water. Cement, along with sand and stone aggregate, make up concrete, the most widely used building material in the world. Steel reinforcing bars (rebars) are commonly added to the interior of concrete for additional strength.
There are many reasons for the popularity of concrete. It is relatively inexpensive, capable of taking on the shape of a mold, has exceptionally high compression strength and is very durable when not exposed to repeated freeze-thaw cycles. However, as a building or construction material, concrete, whether it is reinforced or not, is not without some shortcomings. One major drawback of concrete is that it is relatively low in tensile strength. In other words, it has little ability to bend. Concrete also has little impact resistance and is frequently brittle. A third major drawback is that its durability is significantly reduced when it is used in applications which require it to be exposed to repeated freeze-thaw cycles in the presence of water. Concrete is relatively porous and water is able to permeate the material. Freezing and thawing with the accompanying expansion and contraction of the water, forms cracks in the concrete. Furthermore, if salt is also present in the environment, it dissolves in the water and permeates into the concrete where it is capable of inducing corrosion in any of the rebars or other metallic reinforcements present.
Various techniques have been suggested in the past for overcoming these drawbacks. The addition of fibers to concrete has improved its tensile strength but has decreased its compression strength. Providing exterior coatings on the outer surfaces of the concrete has reduced water permeation, but it is a time-consuming additional step and has little, if any, effect on the lasting strength of the concrete. The addition of modifying agents as freely-mixed additives into a concrete mixture before setting has also been tried. These efforts have met with generally unsatisfactory results. Attempts to add modifying agents in the form of micronodules or prills have also been tried. Frequently, the prills are designed to be heat melted to cause release of the modifying agent into the matrix after setting of the materials. These designs require the application of heat to release the beneficial additive into the matrix after cure. Moreover, the melted, permeated agents leave behind voids in the concrete which weakens the overall structure under load. Accordingly, a demand still exists for an improved concrete matrix material having greater tensile strength, greater durability and comparable or improved compression strength.
In addition to cementitious building materials, the use of polymer composites as structural materials has grown tremendously in recent years. Polymer composite materials have advantages over steel or concrete including good durability, vibration damping, energy absorption, electromagnetic transparency, toughness, control of stiffness, high stiffness to weight ratios, lower overall weight and lower transportation cost. These polymer matrix materials comprise a continuous polymer phase with a fiber reinforcement therein. Some polymer composite materials are three times stronger than steel and five times lighter. They have heretofore been generally more expensive but their use may, in the long term, be economical because of their greatly reduced life cycle costs. Europeans have made bridges completely of specialty polymer matrix composite materials. The polymer composite materials may be used as rebars, tensioning cables, in bonded sheets, wraps, decks, supports, beams or as the primary structures for bridges, decks or buildings. Structures made from polymer matrix materials are specially effective in aggressive environments or are well adapted for building structures where electromagnetic transparency may be needed for highways, radar installations and hospitals.
As used herein, matrix composite materials may refer to generally any continuous matrix phase whether it comprises a settable construction material such as cementitious materials or a thermoplastic material such as asphalt materials, as well as, other synthetic or natural high polymer materials ceramics, metals and other alloy materials. The matrix composite materials include various fiber reinforcements therein distributed throughout the matrix or placed at desired locations within the continuous phase. The matrix composite materials may be fabricated as large building structures and load bearing shaped articles, or they may be molded or machined as small parts for specialty uses. For example, the matrix material may comprise a thin sheet or web of material in the form of a foil, wrap, tape, patch or in strip form. As presently used in this specification, the term matrix composite material does not necessarily refer to large civil engineering structures such as highways and bridges.
In connection with the polymer and/or metal or ceramic matrix composite materials, as well as, in the settable building materials such as concrete materials, special problems cause structures made from these materials to become aged or damaged in use. More particularly, special structural defects arise in use including microcracking, fiber debonding, matrix delamination, fiber breakage, and fiber corrosion, to name but a few. Any one of these microscopic and macroscopic phenomena may lead to failures which alter the strength, stiffness, dimensional stability and life span of the materials. Microcracks, for example, may lead to major structural damage and environmental degradation. The microcracks may grow into larger cracks with time and cause overall material fatigue so that the material deteriorates in long-term use.
Advanced matrix composites used in structural applications are susceptible to damage on both the macro- and microscopic levels. Typical macroscopic damage to composite laminates involves delaminations and destruction of the material due to impact. On the micrographic scale, damage usually involves matrix microcracking and/or debonding at the fiber/matrix interface. Internal damage such as matrix microcracking alters the mechanical properties of shaped articles made therefrom such as strength, stiffness and dimensional stability depending on the material type and the laminate structure. Thermal, electrical and acoustical properties such as conductance, resistance and attenuation have also been shown to change as matrix cracks initiate. Microcracks act as sites for environmental degradation as well as for nucleation of microcracks. Thus, microcracks can ultimately lead to overall material degradation and reduced performance.
Moreover, prior studies have shown that microcracks cause both fiber and matrix dominated properties of the overall composite to be effected. Fiber dominated properties such as tensile strength and fatigue life may be reduced due to redistribution of loads caused by matrix damages. Matrix dominated properties on the other hand such as compressive residual strength may also be influenced by the amount of matrix damage. The impact responses of toughened polymer matrix composites have been studied and it has been shown that matrix cracking precedes delamination which, in turn, precedes fiber fracture. Tough matrices which can reduce or prevent matrix cracking tend to delay the onset of delamination which results in an improved strength composite and longer lasting composite material.
Repair of damages is a major problem when these matrix composite materials are employed in large-scale construction or advanced structures. Macroscale damage due to delamination, microcracking or impacts may be visually detected and can be repaired in the field by hand. Microscale damage occurring within the matrix is likely to go undetected and the damage which results from this type of breakdown may be difficult to detect and very difficult to repair.
In order to overcome the shortcomings of the prior art construction and polymer, ceramic or metal matrix composite materials, it is an object of the present invention to provide new and improved smart structural composite materials having a self-healing capability whenever and wherever cracks are generated.
It is another object of the present invention to provide new and improved composite materials including self-repairing reinforcing fibers capable of releasing chemical agents into the local microscopic domains of the matrix to repair matrix microcracks and rebond damaged interfaces between fibers and matrices.
It is a further object of the present invention to provide a new and improved structural material.
It is another object of the present invention to provide a new and improved cementitious material.
It is still a further object of the present invention to provide a new and improved cementitious or other construction composite material having greater durability and greater tensile strength.
It is still another object of the present invention to provide a new and improved matrix composite materials containing smart self-repairing fiber reinforcement containing repair chemicals therein which may be released by the smart fibers as needed in response to an external stimulus, and optionally which may be refilled with additional repair chemicals as needed in the field.
In accordance with these and other objects, the present invention provides new and improved shaped articles comprising:
a cured matrix material having a plurality of hollow fibers dispersed therein, the hollow fibers having a selectively releasable modifying agent contained therein, means for maintaining the modifying agent within the fibers until selectively released and means for permitting selective release of the modifying agent from the hollow fibers into the matrix material in response to at least one predetermined external stimulus. In accordance with this invention the shaped articles are matrix composite materials of varying size and end use applications. The cured matrix materials have within them smart fibers capable of delivering repair chemicals into the matrix wherever and whenever they are needed.
The present invention also provides a new and improved method for providing shaped articles having long-term durability and environmental degradation resistance comprising the steps of providing a curable matrix composition, distributing a plurality of hollow fibers therein in desired manner so that the hollow fibers are disposed within the matrix material in a desired predetermined distribution. The hollow fibers are filled with a selectively releasable modifying agent therein which is not released during the mixing or distributing step. The fibers are structured so that the modifying agents stay within the interior spaces or cavities of the fibers within the matrix until the matrix is cured or set. After curing, the modifying agents are selectively released from the fibers by application or action of at least one predetermined external stimulus.
In a preferred embodiment, the method of providing a improved durability shaped article comprises providing a cured matrix material containing smart self repair fibers reinforcement therein.
The principles of the present invention are applicable to space age polymer, metal and/or ceramic structural matrix composite materials as well as more conventional cementitious settable or curable building or construction materials.
Other objects and advantages will become apparent from the following Detailed Description of the Preferred Embodiments, taken in conjunction with the Drawings in which: